The built environment plays a critical role in promoting physical activity and health. The association between parks, as a key attribute of the built environment, and physical activity, however, remains inconclusive. This project employs a natural experiment design to assess the impact of the Community Parks Initiative (CPI), a New York City Department of Parks and Recreation (NYC Parks) park redesign and renovation initiative, on physical activity, park usage, psychosocial and mental health, and community wellbeing. The project will use a longitudinal design with matched controls. NYC Parks has identified 134 parks with extreme capital needs in high-priority neighborhoods and will begin renovating parks in waves. In late 2016, the first wave of 35 parks across 55 neighborhoods will close for renovations; these parks will reopen in late 2017. From this initial wave, 20 interventio parks have been selected for inclusion in this study. Intervention park neighborhoods have been matched to a control group of 20 socio-economically similar park neighborhoods that will not be renovated during the study. The study will address two aims. In Aim 1, investigators will compare total volume of PA and other health outcomes among residents in the intervention vs. control park neighborhoods (defined as .25 mile radius around each park) from baseline to 2.5 years post-renovation. We hypothesize that improvements will be observed in PA and health levels in intervention but not control neighborhoods. In Aim 2, an additional 10 park neighborhoods receiving renovation in 2018 and reopening in 2019 will be used to monitor replicability of intervention effects at 1.5 years post-renovation. This combined design enhances our ability to infer causality in a natural experiment. We aim to recruit and retain 40 study participants per park neighborhood, after attrition, for a total of 1600 participants in Aim 1 and 400 in Aim 2. Study participants will represent two distinct sociodemographic strata in each neighborhood: low-income housing and community- engaged residents. Measures include direct park observations and program checklists, park quality, self- reported and GPS-tracked park usage, accelerometry-based physical activity, self-reported psychosocial health, and perceived community wellbeing. The largest natural experiment of its kind to date, this study represents a rare opportunity to provide robust evidence to further our understanding of the complex relationship between parks and physical activity, psychosocial and mental health, and community wellbeing. The findings will inform future investments in health-oriented urban design policies and offer evidence for addressing health disparities through built environment strategies.